The present invention concerns a test adapter according to the generic term of claim 1.
Test adapters are used in series production of loaded and unloaded circuit boards in the field of test and repair and serve for simultaneous contacting of a great plurality of contact locations of the circuit board by an automatic test system. FIG. 3 shows a typical construction of a conventional test adapter. The test adapter comprises a probe plate 1 and receptacles 2 mounted on this plate as well as exchangeable inserted test probes. The test probes 3 have one top called plunger 3a for the contact with test points 7 of a circuit board 6. The circuit board 6 is mounted on a moving plate 4 of the test adapter by the locating pins 5. Usually the circuit board 6 is pressed against the test probes 3 by vacuum or a hold-down technique not drawn in here, the moving plate 4 performing a vertical movement in the direction to the probe plate 1.
Usually in accordance with each test method and size of circuit board the test points 7 assigned to between a few hundred and a few thousand test probes are contacted on the circuit board 6. Here a particularly high economy is offered by the in-circuit-test-method, where, however, all nets of a loaded circuit board have to be contacted each with at least one, in a few cases also several test probes. In order to reach reliable contacting whether special contact surfaces (test pads) are provided on the circuit board in case of nets with only surface-mounted components, or, if possible, existing plated through holes are used. In case of the through hole insertion technique of components decreasing in distribution the connection points of the components can be contacted reliably.
The progressing miniaturization of electronic assemblies requires on the one hand a reduction of the minimum diameter of test pads and plated through holes, which is necessary for the contacting, but on the other hand also the possibility to contact ranges of contact previously not contactable as for instance contact areas of surface-mounted components or thin circuit lines. Just in case of the second requirement very great demands are made on the positioning accuracy.
Here the size of the necessary test contact surfaces depends mainly on the sum of the tolerances of the test probes, on the guidance of the adapter system and on the dimensional stability of circuit board 6. The factors influencing the size of the necessary test surface on circuit board 6 can be subdivided into three groups:
A first error group, which makes up a considerable portion of the total error, is formed by the mounting-caused misalignment of the receptacles 2, which receive the test probes 3, as well as the wobbling clearance of the test probes themselves.
The second important error group concerns the manufacturing or processing-caused deviation of the dimensional stability of circuit board 6. The most important consequences are the misalignment between hole pattern and circuit line pattern, which is directly reflected as misalignment of the test points with regard to the reference holes of circuit board 6, as well as a stretch or shrinkage of the entire circuit board 6 as also of the circuit line pattern 10 with regard to the hole pattern. Furthermore an angular displacement between hole pattern and circuit line pattern forms a source of error of this second error group.
FIG. 2 shows the consequences of the misalignment between hole pattern and circuit line pattern. This misalignment is discernible in FIG. 2 by the non-centric position of the bore hole 24 referred to copper ring 21 surrounding the bore. The same misalignment causes that the drawn-in distance a between mounting hole 20, through which circuit board 6 is fixed on the moving plate 4, and test point 23 shows a deviation and with this a test probe 3 lined up relatively to the center of the receiving hole 20 does not meet the contact point 23 whether at center or does not meet it at all, depending on the size of the contact point.
The third error group results from the guidance of circuit board 6 over the tightly positioned locating pins 5. In order to even out the tolerances of locating pin 5 and receiving bores 20 in circuit board 6 as well as to even out the above named errors of dimensional stability of circuit board 6 the diameter of the locating pins 5 has always be chosen in a manner that a clearance fit is always achieved. Such a clearance fit causes inevitably an positioning error.
It has to be taken into consideration at all sources of error that particularly in case of high production rate circuit boards are got from many different, internationally producing manufacturers, so that close cooperation with the manufacturer in order to reduce tolerances is made more difficult and hardly takes place in practice. Furthermore in an ideal manner automatic or at least easily manageable semi-automatic correction methods are necessary for a running production, because complicated manual correction of occurring tolerances is not compatible with the requirements of the running series production.
Test adapters being aimed in particular at the correction of the above named second error groups, i.e., of the misalignment errors of the board, are known from several patents. In the U.S. Pat. No. 4,820,975 a test adapter with shiftable probe plates is described, where the misalignment is determined by the measuring of reference marks by means of video cameras, and this error will then be adapted automatically over the xy-shiftable probe plate.
An alternative of this solution that requires a lot of technique is the test adapter described later in U.S. Pat. No. 5,321,351, where the locating pins for the circuit board are mounted adjustably and where a justage of each production of circuit boards is also done by the evaluation of reference marks with video cameras.
This test adapter, however, has the disadvantage that the adjusting mechanism for the adjusting pins is only suited to individual adjustments by means of the special adjusting circuit board, so that manual or automatic adaptations within a series of circuit boards cannot be performed. An additional disadvantage results from the locating pins fixed in the probe plate, if, caused by small receiving holes, locating pins with little diameter are needed. In this case the unfavorable relation between diameter and length of the locating pin causes a very low stability. It is a further disadvantage of this version that locating pins fixed in the probe plate abruptly pierce the circuit board at suction of the adapter and thus require additional protection measures to meet the safety regulations.
These three named disadvantages were then removed in the further development according to U.S. Pat. No. 5,408,189. Here the locating pins are tightly fixed in the moving plate, and this can be adjusted continuously over driving elements in relation to the probe plate. In this version, however, the through holes of the spring contact pins through the moving plate must be particularly big, so that for example due to the construction an additional guiding of the plungers in the moving plate is not possible. All three described test adapters are based on the principle that the positions of the circuit board and the one of the receiving bores of the spring contact pins in the probe plate are shifted relatively to each other and that the reference measurement is also carried out between probe plate and circuit board by means of video cameras. Thus the initially described error portions caused by mounting misalignment, wobbling clearance of the probe and clearance of the receiving holes are not taken into consideration and therefore they limit the achievable positioning accuracy. Furthermore for setting up a special adjusting board (golden board) and a complicated setting-up process is necessary, which, for instance, is described in detail in U.S. Pat. No. 5,321,351. Because furthermore the special adjusting plate is not included in the usual schedule of parts supplied by the manufacturer of the circuit boards, considerable additional problems of procuration result possibly.
A further disadvantage at all three named solutions has its roots in the manner of the measurement of misalignment over special reference marks. While with concern to the manufacturing of circuit boards the reference marks described partially in the applications were formerly in the usable inner fringe of the circuit board, in the meantime these reference marks have been moved nearly completely to the outer fringe that is only available in the manufacturing of the circuit boards and is milled off in the completion of the unloaded circuit board. Also the alternative reference marks mentioned in the patent specifications U.S. Pat. Nos. 5,321,351 and 5,408,189xe2x80x94such reference marks for the correction of the component parts are known, for instancexe2x80x94are mostly available only on the top of the circuit board, but very often not on the underside of the circuit board provided most frequently for test contacting. For universal applicability of the method it would be necessary therefore to require on all layouts of boards appropriate reference marks on each contacting side of the adapter. This makes a change of the existing and known design guidelines about circuit boards necessary. To carry through such changes of guidelines is very difficult and lengthy as experience shows.
A further disadvantage of the named solutions results from the necessity of using video cameras at small and therefore spatially very narrow circuit boards. Such video cameras are very expensive in case of small dimensions. Also the light guides with rearrangeable video cameras proposed in the existing solutions are very costly, inter alia because for picture transmission not only one individual optical fibre, but only a fibre bundle is suitable. Separable plug connections for fibre bundles are cost-intensive, too.
For applications in the test of unloaded circuit boards from the German registered pattern DE 29616 272 U1 and from the European application for patent EP 0 859 239 A 2 solutions are technically known, where a correction of the displacement is effected by means of jointly shiftable locating pins. But because in the test of unloaded circuit boards not all error groups are relevant, it is not possible to transfer the solutions to the test of loaded circuit boards. It is one of the most important differences that loaded circuit boards are exposed to an additional heating by soldering, which can lead to stretch, shrinkage or distortion of the loaded circuit board. The locating pins fixed on a joint plate in both solutions are not shiftable from each other independently and therefore they cannot correct a part of the errors. It is a further disadvantage of both solutions that an additional plate would be necessary in the probe field of the test adapter, which reveals in case of use in test adapters for loaded circuit boards considerable disadvantages.
It is the task of the present invention to supply a test adapter, where the present disadvantages are removed and where by simultaneous correction of all important sources of error the positions of the plungers of the test probes for contact with test points on a circuit board can be adjusted more precisely.
According to the invention a test adapter with the features of claim 1 is offered to solve this task, where advantageous arrangements can be taken from the subclaims.
In particular the test adapter according to the invention comprises an adjusting device with means, which can be fixed on the circuit board and which are established adjustably in the moving plate in a direction of a plane that is parallel to the plane of the moving plate, and comprises a measuring device, which on the other hand is mounted on the moving plate and which allows the use of any circuit lines as reference by separate measuring positions for the x- and y- portion of the offset. It is an advantage of this adjusting device of the test adapter that the circuit board is shiftable relatively to the plungers of the test probes and relatively to the moving plate with the passage bores provided for the test probes. By this it is possible to design the through bores in the moving plate with very little clearance, so that a guidance of the test probes through the through bores is possible, what can considerably reduce the wobbling clearance of the test probes. By simultaneous correction of all important errors the contacting result of any circuit board from the running production, which can be easily recorded by means of a measuring microscope, can then be used for the first setting, so that there is no necessity of a special adjusting board. A further advantage results from the mounting of the locating pins in the moving plate, because this way a sufficient stability is ensured by the little length of the locating pins also in case of a little diameter.
In an advantageous manner the means comprise two locating pins, each one end of which can engage with the bore holes of the circuit board and each other end of which is established in the moving plate adjustably by means of an adjusting mechanism. Here one locating pin can be movable by means of the adjusting mechanism in two vertical directions parallel to the plane of the circuit board, and the other receiving pin can be movable by means of the adjusting mechanism in one direction and can be movable in the direction vertical to it being guided by a spring-loaded element. It is advantageous in this version that the clearance of the locating pins can be evened out in the receiving holes of the circuit board, so that errors of the third error group named before can be corrected.
Further in accordance with the invention the movement of the adjusting device can be accomplished by means of a motor, and control can be provided for the motor, which controls the motor, dependent of signals of the sensors, so that the adjusting device is adjusted in an automatic control loop. It is advantageous in this version that errors of every circuit board can be recorded by means of the sensors, and the adjusting device can be adjusted automatically by means of the motor. Furthermore the movement of the adjusting device can be also effected additionally or aloneover adjusting hand wheels provided manually with scales. This version allows a particularly easy and reproducible adjustment of the circuit board relatively to the test wheels. Thus in this case the locating pins of the adjusting device are not adjustable only one time, but they can be re-adjusted continuously.
In a further version of the test adapter according to the invention guide receptacles are provided for the test probes in the passages of the mounting plate. Furthermore a guide plate can be arranged on the side of the mounting plate, which faces the circuit board. This guide plate has guide bores at the through bores of the moving plate. By these measures errors of the first error group (mounting tolerances and wobbling clearance of the test probes) can be corrected.
The sensors of the test adapter according to the invention can be sensors, which sense for one, at least one direction the position of a line or line edge. These can be movable point sensors or movable optical reflex sensors, where the movement of the sensors can be carried out through the movement of the probe plate towards the circuit board. Such point and reflex sensors are spatially very small and considerably more economical than picture picking-up devices, which pick-up a reference picture. By the movement of the sensor and the mathematical evaluation of the signal of this sensor measuring errors can be detected, and at the same time changes of the optical parameters as for instance another shade of the solder resist or another base material of the circuit board can be detected automatically and can be taken into consideration.
Another version of the sensors results from conducting surfaces tightly mounted on the circuit board supporting plate, so that the offset can be evaluated over electrical parameters as capacity, inductivity or capacitive and inductive coupling between a circuit line of the circuit board and the surface fixed on the test adapter.
In an advantageous manner the sensors are arranged near the center of the circuit board in each case. It is the advantage of this arrangement that along with the offset of the hole pattern with regard to the pattern of the circuit line an optimal adaptation to stretch and shrinkage of the circuit board can be effected. In particular an optimal correction can be achieved by this in case of linear expansion or stretch.
Furthermore by means of the sensors of the test adapter according to the invention certain circuit lines (tracks) or circuit line edges of the printed circuit wires of the circuit board may be detectable locally. For this the sensors and the adjusting device are connected in an advantageous manner with an evaluation device, which compares the positions of certain circuit lines, circuit line edges of the circuit board, detected by the sensors, with the corresponding theoretical positions and so determines the offset of the real circuit line pattern and transfers it to the adjusting device.
By the comparison of the really detected positions of certain parts of the switching arrangement with the theoretical positions, which may result from the CAD data for instance, it is possible to determine the offset without providing special reference marks and to correct it therefore. Furthermore by the fact that the sensors measure circuit lines (tracks) or circuit line edges of the printed circuit board, nearly any quantity of optical sensors can be mounted on freely eligible places without special reference marks being provided on the circuit board at these positions. By this further errors as for instance angular errors can be optionally detected and corrected or several positions can be measured also in case of circuit boards with great dimensions, and so the test of the left half of the circuit board can be carried out with other correction than that of the right half of the circuit board, for instance.
Furthermore the test adapter according to the invention can comprise three sensors for the detection of the offset and for the turn of the circuit line pattern to the bore holes.
Furthermore a test adapter according to the invention is offered, where two test adapters according to the invention are arranged on both sides of the circuit board to be tested, each of the adjusting devices making possible an independent setting of the test adapter with regard to each of the switching arrangements on each of the sides of the circuit board. The present invention is now illustrated with embodiments with reference to the drawings.